The ability of a limited metabolic assessment to identify pediatric stone formers with metabolic abnormalities

American Urological Association guidelines recommend a urinary metabolic evaluation after the first stone event in all pediatric stone patients. Prior studies identified hypercalciuria and urine hypovolemia as the most common abnormalities in children with urolithiasis. Recent data suggest that hypocitraturia is most prevalent. It was hypothesized that a limited evaluation would detect the majority of clinically significant metabolic abnormalities in pediatric stone formers.

A retrospective analysis of all children (<18 years of age) with renal/ureteral calculi evaluated at the study institution from 2005 to 2015 was performed. Children with ≥ one 24-h urinary metabolic profile after a clinical visit for renal/ureteral calculi were included. Those with bladder stones and those with undercollection or overcollection or missing urinary creatinine were excluded. Demographics and data from the first urinary metabolic profile and stone analyses were collected. The sensitivity, specificity, and positive and negative predictive value (NPV) of a limited urinary metabolic evaluation consisting of four parameters (24-h calcium, citrate, and oxalate and low urinary volume) were compared to a complete urinary metabolic profile. The number and type of metabolic abnormalities that would have been missed with this limited evaluation weredetermined.

Of 410 patients, 21 were excluded for age ≥18 years, 13 for bladder stones, 248 for overcollections, 38 for undercollections, and 10 for missing creatinine. This left 80 patients for inclusion: median age 11.4 years, 60% female, and 96.3% white. Of the entire cohort, 69.6% had hypocitraturia, 52.5% had low urine volume, and 22.5% had hypercalciuria. Sensitivity was 87.5%. Specificity could not be calculated because no patients had a normal complete metabolic evaluation. The NPV was zero, and the positive predictive value was 100%, but these are artifacts resulting from the absence of patients with a normal complete metabolic evaluation. Of the 80 patients, 10 had at least one abnormality missed by a limited metabolic evaluation (Table 1). The missed abnormalities were high pH (n = 6), abnormal 24-h phosphorus (low in 1 patient and high in 1 patient), low 24-h magnesium (n = 3), low 24-h potassium (n = 3), and high 24-h sodium (n = 4).

A limited urinary metabolic evaluation would have detected the vast majority of clinically significant metabolic abnormalities in the study sample. Approximately two-thirds of the study patients submitted inadequate 24-h urine specimens.

A simplified approach to metabolic evaluation in first-time stone formers with a stone analysis available was proposed. This streamlined approach could simplify the metabolic evaluation and reduce health care costs.